In heavy oil upgrading, heavier materials are converted to lighter fractions and most of the sulfur, nitrogen and metals must be removed. Heavy oils include materials such as petroleum crude oil, atmospheric tower bottoms products, vacuum tower bottoms products, heavy cycle oils, shale oils, coal derived liquids, crude oil residuum, topped crude oils and the heavy bituminous oils extracted from oil sands. Most residual oil feedstocks which are to be upgraded contain some level of asphaltenes which are typically understood to be heptane insoluble compounds. Asphaltenes are high molecular weight compounds containing heteroatoms which impart polarity.
Heavy oil is upgraded in a primary upgrading unit before it can be further processed into useable products. Primary upgrading units known in the art include coking processes, such as delayed or fluidized coking, and hydrogen addition processes such as ebullated bed or slurry hydrocracking (SHC). In SHC, a three-phase mixture of heavy liquid oil feed cracks in the presence of gaseous hydrogen over solid catalyst to produce lighter products under pressure at an elevated temperature. During an SHC reaction, it is important to minimize coking. Under certain process conditions, the asphaltenes can self-associate, or flocculate to form larger molecules, generate a mesophase and precipitate out of solution to form coke. Mesophase is a semi-crystalline carbonaceous material defined as round, anisotropic particles. The presence of mesophase can serve as a warning that operating conditions are too severe in the SHC reactor and that coke formation is likely to occur under prevailing condition.
The formation of mesophase particles is typically monitored by labor-intensive laboratory analysis that entail several hours of delay to actual process events. Industry is in need of a fast, on-line method of monitoring the volume fraction and size distribution of mesophase particles.